1. Field of the Invention
This invention relates to a magnetic head for analog recording of video images in a home VTR such as S-VHS, high definition video system, etc.
2. Description of Related Art
It has been a recent trend in a home VTR to record digital signals in a wider band of frequencies for the purpose of improving the image quality. For example, the central frequency of a luminance signal has been changed from 4.3 MHz to 6.5 MHz (S-VHS) in the VHS format.
In consequence, recording has been made with high density with the shortest recording wavelength of 1 .mu.m or shorter. So that the coercive force of a recording medium has been increased and a magnetic head has been formed with a narrower gap. On the other hand, a playback output in a lower frequency by the home VTR (the central frequency of a color signal is 0.829 MHz in S-VHS) holds the key of recording since the color signal is superimposed upon a luminance signal for recording. Perfect recording could not be expected in a conventional ferrite head having a narrow gap, and a sufficient playback output was not obtained in the lower frequency. With respect to this problem, a metal-in-gap head (referred to as a "MIG head" hereinafter) has brought in use, which has a metal film with high saturation flux density, such as Co amorphous or sendust only in the vicinity of the gap of the MIG head where the magnetic saturation easily occurs in recording, while the most of the core of the MIG head is formed of ferrite. FIG. 1 is a plan view of the rubbing plane in touch with a magnetic tape, of a conventional MIG head and disclosed in Japanese Patent Application Laid Open No. 60-95704. In FIG. 1, a reference numeral represents a gap a reference numeral 2, a metal film having high saturation flux density and formed of, e.g., Co amorphous or sendust, a reference numeral 3 a magnetic core half formed of a magnetic compact with high permeability, for example, Mn-Zn ferrite, and a reference numeral 4 a bonding glass.
FIGS. 2 and 3 show plan views of the rubbing plane of a conventional MIG head disclosed in Japanese Patent Application Laid Open Nos. 60-32107 and 58-1820, respectively. These MIG heads are effective particularly to a high-coercivity medium (having the coercive force of about 1500 Oe) such as a metal particle tape but sufficiently effective to an S-VHS tape (having the coercive force of about 900 Oe).
According to the S-VHS system, the relative speed between the head and the magnetic tape is 5.8 m/s, the maximum recording frequency is approximately 8 MHz where the wavelength is about 0.73 .mu.m. However, in such a short wavelength as above, the spacing loss easily influences the head. Due to the difference in the wear characteristics against the tape between the ferrite and the metal film, the surface of the metal film 2 becomes hollow relative to the magnetic core halves 3 formed of ferrite, thereby generating a spacing d, as shown in FIG. 1. When the metal film 2 is formed of sendust, the actual spacing d is about 200 .ANG.. The spacing loss Ls is expressed by a following equation (1); EQU Ls=54.6(d/.lambda.) (dB) (1)
wherein d is the spacing between the head and tape, and .lambda. is the wavelength. Accordingly, the spacing loss Ls is about 1.5 dB.
Moreover, the thermal coefficient of the metal film 2 is greatly different from that of the ferrite. Therefore, when the metal film 2 is several tens .mu.m thick, the metal film easily separates from the magnetic core halves.
The track width is approximately equal to the film thickness of the metal film 2 in the MIG heads with the structure shown in FIGS. 1 and 2. Accordingly, it is difficult to produce the head with good yield when the film thickness should be about 50 .mu.m for a head, e.g., in the standard mode of the S-VHS (58 .mu.m track pitch).
On the other hand, although the film thickness of the metal film 2 is not dependent on the track width in the structure of the MIG head indicated in FIG. 3, the boundary between the ferrite and metal film 2 functions as a pseudogap, that is, a pseudogap effect, allowance of the pseudogap is little in analog recording, and therefore it is practically difficult to mass-produce the head.